An HTPC has a lot of work to do from scaling video to running a software DVD player. More and more people are using Windows XP as the OS of choice for their HTPCs. The standard installation of XP installs and runs a lot of services (memory resident programs) to support the standard home or business user. Many of these services are not needed or used by someone using XP for a HTPC. Most of these services can be disable from the Services menu under Control Panel. This link provides a complete list of all of the services under XP, their function, and recommended settings for each service.
HTPC Performance - Windows XP Services Removal Guide